A fan looks at a shop on Diagon Alley at the Warner Bros Harry Potter studio tour in Leavesden, Hertfordshire. Photograph: Frantzesco Kangaris

JK Rowling has sprung a surprise on her fans by opening the doors to her long-awaited Pottermore website without any warning.

A select million fans were granted access to the site last summer following a hunt for a 'magic key' hidden on a series of websites, including the Guardian. As opening to millions more frustrated devotees was delayed, speculation grew that the developers were finding it hard to make the magic work, even with a limited number of entrants.

Analysts will be keeping a close eye on the site on Saturday to see how it holds up under pressure, which could amount to hundreds of thousands of visitors a minute.

A press release issued just before 9am on Saturday said: "JK Rowling's unique online experience Pottermore.com, based on her hugely popular Harry Potter books and partnered by Sony, is opening from today, following its closed Beta testing phase."

"Many more Potter fans old and new will now be able to explore, discover and share the stories in a completely new way. At Pottermore.com, they will be able to join Hogwarts School of Witchcraft and Wizardry, get sorted into one of the four houses, and have a wand choose them, before starting a journey through the storylines of these extraordinary books and discovering exclusive new writing from J.K. Rowling."

It added that Pottermore.com would begin with Harry Potter and the Philosopher's/Sorcerer's Stone, "but will rapidly grow and develop, as new content, features and functionality are added. Users will be able to share their experiences and creativity as the storylines move on to subsequent books in the series in the months following opening."

Members of the Guardian children's books website who managed to get hold of a magic key have reported a mixed experience, with Shoshana Kessler writing:

"Once you're in, a purple screen greets you. It's fronted by an intricate design depicting a gate with padlocks on the top. There are seven circles cast within the gate, after each book. I have to admit, it's incredibly exciting.

"I click on the shield at the top, which takes me to a homepage of sorts. There are sections for your wand, your house points, notifications, friends and favourites. Most exciting are the sections for Spells and Potions, showing you 'how to manage your potions'.

"I learn that Number 4 Privet Drive is based on JK's childhood house (I did already know that, but that's only because I'm an über fan). I move forward, and see pictures of key moments in Chapter 1, each beautiful and exciting but with not much else …"

Responding to the article, Brenda wrote: "I usually pop in a couple of times a week to see if my Ravenclaws (Claws) are higher than the Slytherin in points. I have brewed a couple of potions but I usually don't stay online long enough. Duelling wasn't what I expected either having had the first four video games on my PC."

Chris expressed a frustration that looks set to be shared by many more fans until the site develops past its Philosophers' Stone phase. "After completing the Philosophers Stone I was raring for more knowledge of the Potter-world but I was sadly cut off after it," he wrote. Pottermore.com is opening in English (UK and US editions), French, Italian, German and Spanish.

JK Rowling has finally made good on her promise at last summer's launch, "to give something back to the fans", but what do you think? Tell us in the comments below.